Control and servo systems, such as hydraulic or pneumatic systems, are well known and operate on the simple principle of transferring force from an applied location to an output location by means of an incompressible fluid. The transfer is typically accomplished by means of a cylinder having a piston contained therein pushing the incompressible fluid through a fluid line to another cylinder, also having a piston, at a different location. One tremendous advantage to transferring force through a hydraulic system is that the incompressible fluid line connecting the two cylinders can be any length and shape, and can wind or bend through all sorts of positions separating the two pistons. The fluid line can also split into multiple other fluid lines thus allowing a master piston to drive multiple slave pistons. Another advantage of hydraulic systems is that it is very easy to increase or decrease the applied force at the output location. This hydraulic force multiplication is accomplished by changing the size of one piston relative to the other.
In most hydraulic systems, cylinders and pistons are connected through valves to a pump supplying high-pressure hydraulic fluid as the incompressible fluid. Spool valves are the most commonly used valves in hydraulic systems and can apply both forward and backward pressure to hydraulic actuators. Usually, in a piston type actuator, drawing the piston back into the cylinder requires very little force and can occur relatively quickly. To accomplish this, the highest possible flow rate of fluid at low pressure is desired and can be realized by moving the spool valve to a position that opens a return fluid line. When pushing the piston, however, the highest possible pressure is necessary in order to generate the maximum force at the output end. Spool valves are ideally suited to hydraulic systems because they allow manipulation of the flow rate to achieve hydraulic force.
Still, in spite of the advantages of spool valves in hydraulic systems, existing spool valves have certain design limitations. Traditional spool valves have been designed to be actuated by either electrical servos or internal control pressures called pilot pressures. Spool valves are commonly mounted in a cylindrical sleeve with fluid ports extending through the sleeve which can be opened or closed for fluid communication with each other by positioning the lands and recesses of the spool in appropriate locations within the sleeve. The working pressure is varied by opening or closing the valve allowing more or less pressurized fluid to flow from the reservoir. Usually, the valve is controlled by an electrical current. The current is related to the pressure in that the greater the current supplied, the wider the valve gate is opened allowing more pressurized fluid to flow. When the load pressure in the actuator finally equals the supply pressure then flow stops. In other words, for a given current you get a prescribed flow and when you get a load pressure that equals the supply pressure then the flow drops off and finally stops. As the load pressure approaches the reservoir pressure the valve loses linear response and the spool valve device system becomes unstable. Consequently, spool valve devices are typically run in the regime where the pressure source (i.e. reservoir) is very high compared to the load pressures and the flow versus input current linear in the usable region. This means that the system, and particularly the load, is always in a pressurized state and cannot be freely moved by an external force or under its own weight.
In addition to the current flow problems of traditional spool valves, classical hydraulic systems are problematic for several other reasons. First, complex controllers are needed to control the cycle times of valves and pistons. Second, cycle times for moving pistons are often long because large amounts of fluid are required to move output pistons. Third, the large quantity of fluid needed to drive output pistons requires constant pressurization of large reservoirs of fluid. Consequently, hydraulic machines typically require large amount of hydraulic fluid for operation and therefore require large external reservoirs to hold the difference in the volume of oil displaced by the two sides of any cylinder.
Classical spool valve devices are also limited in application because when a controlled flow is induced through a valve it is usually only translated into a controlled velocity. Consequently, complex system feedback devices must be used to convert the energy from a velocity system to a position system. Introducing feed back devices into the system limits the response of the system to the bandwidth of the feedback loop and the valves such that the time delays between the feedback devices and the valves make the system unstable when a resistive force is applied.
Still other problems exist with classical servo valves operating in classical servo systems. Due to the problems discussed above, these valves and systems are incapable of performing at high bandwidths without going unstable. In addition, significant amounts of energy may be lost when not all of the valves in a multiple valve system are being used. Finally, these systems exhibit poor impedance properties.